Great, couple statements / questions:
My understanding is the octane rating of a fuel is ignition temperature and doesn’t effect burn speed… of course different fuels will have different burn speeds, but NOT directly related to octane rating. Each mixture will have different peak pressures at X timing advance, but MBT will always be 15ish degrees. I think the peak pressure prior to TDC is somewhat unimportant and the focus should be MBT. Burn rates will effect pressure BTDC, but the slope average will be similar generally speaking.

Thanks for clearing up the water discussion, that vaporization will happen before spark… thus the claim that water absorbs EGT is false correct and any effect on EGT is related to tuning only? This brings up another question: during combustion, energy is created based on A/F mixture/burn percentage and NOT pre-mixture environment… so temp pre-combustion does not directly relate to temp post-burn. Efficiency of combustion is the only determinant of EGT (ie. What energy is used for work versus heat)?? Hope my question is clear.

My understanding is the octane rating of a fuel is ignition temperature and doesn’t effect burn speed… of course different fuels will have different burn speeds, but NOT directly related to octane rating.

I would like clarification on this as I remember reading about guys who went for too high of an octane rating and it was the slow burn speed that affected them negatively.

Great, couple statements / questions:
My understanding is the octane rating of a fuel is ignition temperature and doesn’t effect burn speed… of course different fuels will have different burn speeds, but NOT directly related to octane rating. Each mixture will have different peak pressures at X timing advance, but MBT will always be 15ish degrees. I think the peak pressure prior to TDC is somewhat unimportant and the focus should be MBT. Burn rates will effect pressure BTDC, but the slope average will be similar generally speaking.

Thanks for clearing up the water discussion, that vaporization will happen before spark… thus the claim that water absorbs EGT is false correct and any effect on EGT is related to tuning only? This brings up another question: during combustion, energy is created based on A/F mixture/burn percentage and NOT pre-mixture environment… so temp pre-combustion does not directly relate to temp post-burn. Efficiency of combustion is the only determinant of EGT (ie. What energy is used for work versus heat)?? Hope my question is clear.

I don't mean to be a jerk, but most of what you've posted above is not true....at least the way I read it.

I would like clarification on this as I remember reading about guys who went for too high of an octane rating and it was the slow burn speed that affected them negatively.

If I "owned" the data I could show it, but for now all I can do is talk about it in an anonymous way.

A pressure trace of combustion shows the answer to every question in this thread.

Edit: Burn speed also changes with pressure and heat. You can run a high octane fuel that burns slower in a 10:1 engine, and it loses power. Then you increase the compression ratio to 12:1 and it makes more power. The increased compression has increased burn speed, but with the detonation resistance of the higher octane fuel, you can run the timing in the same place or further advanced than with a 10:1 engine on pump fuel. The result is that less of the burn happens before TDC. This is one of the keys to power.

The other thing that many people miss is burn consistency. I've seen pressure traces where there is up to a 30% deviation in cylinder pressure between the minimum and maximum combustion event. You have to tune for the maximum pressure, so you end up missing the optimum tune for the minimums. A lot of power is made by making the burn more consistent...not necessarily making more peak cylinder pressure.

I don't mean to be a jerk, but most of what you've posted above is not true....at least the way I read it.

well it was kinda 2 long questions and I think I used some terms incorrectly. Substitute peak pressure for MBT and visa versa... there, fixed.

I'm sure fuel subject can get very complicated, but generally octane rating is ignition temp. For different octane ratings you have to use different fuels (or additives) which will have different burn properties.

the question on EGT, I'm very curious about... i tried to rewrite it, but i don't have the patients. i give up.

You can run a high octane fuel that burns slower in a 10:1 engine, and it loses power. Then you increase the compression ratio to 12:1 and it makes more power. The increased compression has increased burn speed, but with the detonation resistance of the higher octane fuel, you can run the timing in the same place or further advanced than with a 10:1 engine on pump fuel. The result is that less of the burn happens before TDC. This is one of the keys to power.

There we go, so the compression ratio is a factor as well in the burn speed depending on the octane.

The company that builds the laboratory engine that is used to test Motor Octane is based there.

Every fuel octane rating in North America is made using this one cylinder engine made by this company that I visited. I had to learn what it was about.

Originally Posted by Matt@Camber-Toe

its a little over my head but how do you go about increasing burn consistency? Say in an old M30 engine with no electronic gizmos to alter cam timing or individual cylinder fuel injection..

It's a little over my head too, because I haven't done this yet myself. That is, go through the process of finding the burn problem, and solving it. The people that I have talked to about this have said it's a black art, that there are lots of things people try to solve this. The basics comes down to instrumentation though, and making sure everything is working properly. One of the main areas I've been told about is ignition timing drift. Lots of products have drift, some greater than others, but there is no standard to test by, and the industry in general tries not to get into it. Quite simply, there are lots of ECUs that have drift in them, but work fine with general applications. Having 1-2 degrees of drift on a 4000hp engine is a problem though....so thats where you'll see specific ECUs in use...and people generally think they are over-priced. Depends on the application. I've seen with the nitrous guys specifically that the are trying to smooth out the distribution system. I've got high speed video of a nitrous nozzle "A" sputtering due to turbulance internally prior to the nozzle, and nozzle "B" have a more even flow. The sputtering when you are introducing fuel at the same point will vary how the fuel is dispersed...and that will create a deviance from combustion event to event.

With meth, that's one thing I've consistently heard is that the PCP doesn't seem to change, but the average of PCP changes. That allows a more advanced ignition point...and around 8% more power on a well setup system. That's as a primary fuel though.

When you are adding meth in addition to the primary fuel, I think the main benifits are the 4 areas I posted above about. I don't know how the burn consistency changes, as I haven't tested that yet.

One of the really big down sides to using pure meth, is that it really doesn't like epoxies. That's a key tool of quality head porters....that re-shape the port by adding expoxy. I've got on on the 330i project with epoxy added...so I'm a little nervous about how much meth I can run. The head guys all say I should be all right because I'll run pure gasoline most of the time, and it will wash the meth away, but it's still in the back of my head.

The company that builds the laboratory engine that is used to test Motor Octane is based there.

Every fuel octane rating in North America is made using this one cylinder engine made by this company that I visited. I had to learn what it was about.

It's a little over my head too, because I haven't done this yet myself. That is, go through the process of finding the burn problem, and solving it. The people that I have talked to about this have said it's a black art, that there are lots of things people try to solve this. The basics comes down to instrumentation though, and making sure everything is working properly. One of the main areas I've been told about is ignition timing drift. Lots of products have drift, some greater than others, but there is no standard to test by, and the industry in general tries not to get into it. Quite simply, there are lots of ECUs that have drift in them, but work fine with general applications. Having 1-2 degrees of drift on a 4000hp engine is a problem though....so thats where you'll see specific ECUs in use...and people generally think they are over-priced. Depends on the application. I've seen with the nitrous guys specifically that the are trying to smooth out the distribution system. I've got high speed video of a nitrous nozzle "A" sputtering due to turbulance internally prior to the nozzle, and nozzle "B" have a more even flow. The sputtering when you are introducing fuel at the same point will vary how the fuel is dispersed...and that will create a deviance from combustion event to event.

With meth, that's one thing I've consistently heard is that the PCP doesn't seem to change, but the average of PCP changes. That allows a more advanced ignition point...and around 8% more power on a well setup system. That's as a primary fuel though.

When you are adding meth in addition to the primary fuel, I think the main benifits are the 4 areas I posted above about. I don't know how the burn consistency changes, as I haven't tested that yet.

One of the really big down sides to using pure meth, is that it really doesn't like epoxies. That's a key tool of quality head porters....that re-shape the port by adding expoxy. I've got on on the 330i project with epoxy added...so I'm a little nervous about how much meth I can run. The head guys all say I should be all right because I'll run pure gasoline most of the time, and it will wash the meth away, but it's still in the back of my head.

Just something else to consider.

True. Guess for my average 3.5L making 3-400hp it isnt much of a concern then?